1. Field of the Invention
The present invention relates to a defect inspection apparatus and method for inspecting defects existing on an inspection-target object. More particularly, it relates to a defect inspection apparatus and method used for observing and inspecting defects and foreign substances on microscopic patterns in fabrication lines such as semiconductor fabrication steps and flat-panel display manufacturing steps.
2. Description of the Related Art
In the fabrication lines such as semiconductor fabrication steps and flat-panel display manufacturing steps, the observation and inspection of the defects and foreign substances on microscopic patterns are performed using optical microscopes. From the viewpoint of illumination schemes, the microscope optics used for the defect inspection like this are classified into two types, i.e., bright-field illumination scheme and dark-field illumination scheme. Here, the bright-field illumination scheme is a scheme which takes in both specular reflection light of illumination light and scattered/diffracted light thereof by using an objective lens. Meanwhile, the dark-field illumination scheme is a scheme which, without taking in the specular reflection light, takes in only the scattered/diffracted light by using the objective lens. In general, the bright-field illumination scheme is superior in the defect detection sensitivity; whereas the dark-field illumination scheme is suitable for speeding up the inspection speed. As a result, in many cases, the bright-field illumination scheme is mainly used for inspecting more microscopic defects; whereas the dark-field illumination scheme is used for high-speed 100-% inspection. Incidentally, in the high-speed 100-% inspection, a higher priority is given to the processing speed rather than to the highest sensitivity. In recent years, however, the defects to be inspected have become more and more microscopic in accompaniment with the enhancement in the integration scale of semiconductor devices. This situation has required that even higher performance for the microscope optics be implemented in defect inspection apparatuses of the bright-field illumination scheme as well.
Resolving power of an optical microscope is determined in principle by wavelength of light to be used for image-formation and numerical aperture (NA) of an objective lens. Accordingly, so far, higher-performance implementation for the optical microscope has been accomplished as the higher resolving-power implementation based on shorter-wavelength implementation for the wavelength of light to be used for image-formation and higher numerical-aperture (NA) implementation for the objective lens. These implementations enhance the resolving power of the optical microscope, thereby enhancing gray-scale contrast between both of microscopic patterns and microscopic defects existing thereon, and thus making it easier to detect more microscopic defects. In the status quo, however, the shortest wavelength of a light source usable for the optical microscope is equal to about 200 nm at best. Namely, there exists a limit to the enhancement in the resolving power which is based on the shorter-wavelength implementation. Also, as the numerical aperture (NA) of the objective lens, an objective lens whose numerical aperture is high enough, i.e., 0.8 to 0.9, has been already used in the bright-field illumination scheme. This high numerical-aperture value is substantially the limit as a dry-type objective lens. Moreover, even if an immersion-type objective lens is used together with a liquid whose refractive index is equal to 1 or more, the numerical-aperture value capable of being implemented is equal to about 1.5 at the maximum. Consequently, the enhancement in the resolving power remains at only about 1.6 times larger as compared with the case of the dry-type objective lens at present.
In view of this situation, as a technology for enhancing the resolving power by using a method other than the shorter-wavelength implementation for the light source and the increase in the objective-lens numerical aperture, there exists a method of using the super-resolution technology as disclosed in JP-A-2000-155099. Spatial frequency itself for the resolution limit is determined by the wavelength to be used and the objective-lens numerical aperture. Accordingly, this method cannot improve the spatial frequency itself for the resolution limit. Instead, this method suppresses low spatial-frequency band in the transfer function (MTF) of the image-formation system, and lifts up the high spatial-frequency band relatively. This operation enhances the gray-scale contrast between both of the microscopic patterns and the microscopic defects existing thereon, thereby making it easier to detect more microscopic defects.
In the defect detection using an imaging device, a light-amount range which the imaging device is capable of detecting, i.e., the dynamic range, is finite. As a result, the defect detection capability is generally determined by gray-scale contrast of the ratio between a light-amount difference caused to occur on the image at a defect position by the presence or absence of the defect, and a light amount at which output signals from the imaging device become saturated. The methods explained in the conventional embodiments, i.e., the shorter-wavelength implementation for the light source, the increase in the objective-lens numerical aperture, and the super-resolution technology, enhance the gray-scale contrast between both of the microscopic patterns and the microscopic defects existing thereon, thereby making it easier to detect more microscopic defects. These methods have found it possible to enhance the detection sensitivity for the microscopic defects because of the following reason: Namely, when an optical-microscope image of an inspection-target object is acquired, if low spatial-frequency structures on the inspection-target object, such as a non-pattern portion and a wide-area pattern portion, have become the brightest portions on the optical-microscope image, the gray-scale contrast of the microscopic defects will be enhanced while the brightness of the low spatial-frequency structures will be suppressed relatively. However, if the microscopic patterns, which are also high spatial-frequency structures as is the case with the microscopic defects, have become the brightest portions, the gray-scale contrast of the microscopic defects will be enhanced. At the same time, however, the gray-scale contrast of the microscopic patterns will also be enhanced simultaneously. Consequently, there has existed a problem that it is impossible to enhance the detection sensitivity for the microscopic defects further than that.